Verified answer

Because its cool? Because the photon loves the neighborhood of the nucleus?

http://en.wikipedia.org/wiki/Pair_production

The best answer might be that it isn't forbidden. Murray Gell-Man noted "Whatever isn't forbidden is required, thus, if there's no reason why something shouldn't exist, then it must exist.

http://lawsoflife.co.uk/gell-manns-dictum/